An Adjustable-Rate User-Printable Rain Gauge Calibrator

Abstract. Accurate precipitation measurement is essential. However, calibration of field-deployed rain gauges remains a challenge. Many methods require laboratory conditions, costly commercial equipment, or instruments designed for specific rain gauges, which cannot accommodate smaller-diameter gauges.

We introduce the Adjustable-Rate 3D Printed Rain Gauge Calibrator (AR3D), a low-cost, open-source device designed for in situ calibration of rain gauges.

The AR3D introduces three innovations: (1) an adjustable screw valve, enabling flow rate tuning from 0.15–16 mL min⁻¹, accommodating the lower flow rates needed for smaller rain gauges; (2) a compact and durable design developed with low-volume reservoirs and elimination of degradable parts; and (3) an integrated pyranometer cover to generate automatic calibration event signals.

We evaluated the AR3D using gravimetric tests, constant-rate stability tests, device-to-device equivalence tests, and field comparisons. Laboratory gravimetric tests of the AR3D demonstrate its ability to deliver volumes of water accurately with an average error of 0.11 % (comparable to ISO Class B tolerances for plastic volumetric flasks) at flow rates within ±5 % equivalence across independent devices, with a coefficient of variation (CV) below 5 %. The AR3D has been successfully deployed to weather stations in both Kenya and the United States. Field validation of the AR3D with a single setting resulted in a flow rate CV of 8.7 %. The AR3D enabled identification of properly functioning rain gauges, as well as an under-reporting station. The total cost of material and labor to build an AR3D is approximately USD 12.